1. Field of the Invention
This invention relates to cooling of plastic molding machines, and more particularly to an apparatus and method for liquid overfeed refrigerant cooling of the molds of molding machines for the manufacture of plastic articles.
2. The Prior Art
It is known that to improve the efficiency, throughput, and product quality of plastic molding machines, it is desirable to provide a means for cooling the mold. Such mold cooling brings about a more rapid solidification of the hot liquid plastic, shortening the time that must be waited before the freshly molded article develops sufficient strength to be removed from the mold. Furthermore, coolant flow passages in the mold may be selectively arranged to cause evenly distributed cooling according to the shape of the molded article to achieve a product of uniform physical properties. Consquently, the development of methods and apparatus for achieving effective and efficient mold cooling has been a concern of those skilled in the plastic molding art.
As a basis for attempting to achieve improved plastic mold cooling, it is important to understand the factors involved in mold heat transfer. These factors include the thermal conductivity of the plastic itself, the specific heat of the plastic itself, the heat transfer characteristics of the boundary layer between the plastic and the inner surface of the mold, the thermal conductivity of the mold material, the variable linear distance between the inner surface of the mold and the coolant flow passage, the total mass of the mold, the heat transfer characteristic of the boundary layer between the surface of the cooling channel and the cooling medium, the temperature of the cooling medium, the velocity of the cooling medium, the temperature of the hot plastic, and the radiation effect of the mold to objects within the space.
Conventional mold cooling systems presently in use employ water as the coolant medium, a liquid having relatively high boiling and freezing points. The temperature of the coolant is lowered by passing it through refrigerated coils, a process which is low in energy efficiency. To increase the rate of mold cooling, much emphasis has been placed in the past on the importance of temperature and velocity of the cooling medium. As a result, the coolant temperature has been lowered, and the coolant flow rate has been increased, or both. Lowering the coolant temperature results in a greater temperature difference between the coolant and the inner surface of the mold coolant passages. Increasing the coolant velocity tends to increase the rate at which heat is carried away from the mold.
In a typical water-cooled injection molding situation, the hot plastic will be injected at a temperature of about 480.degree. F. Water coolant is typically at 50.degree. F. According to the basic heat transfer equation, the rate of heat transfer is equal to the overall coefficient of heat transfer times the product of temperature difference and the area of the cooling interface. By lowering the water temperature from 50.degree. F. to 40.degree. F., thus increasing temperature difference from 430.degree. F. to 440.degree. F., the rate of overall heat transfer is increased by only about two percent. This small gain is more than offset by the added cost of further cooling the water and the possible need to dehumidify the room.
Of course, the water temperature should not be below the freezing point. When water is cooled from 42.degree. to 40.degree. F. in a typical heat exchanger with refrigerant, the refrigerant must be at about 30.degree. F. Therefore, to prevent icing, an antifreeze such as glycol must be added to the water. However, this changes the specific heat, viscosity and conductivity of the heat transfer fluid thereby reducing overall mold cooling efficiency.
The approach of increasing the water coolant flow rate is effective in reducing overall costs only up to an optimum point, after which, exponentially increasing costs of pumping more than offset the savings from the increased heat transfer rate.
One of the impediments to rapid and efficient heat transfer is the existence of a resistance to heat transfer known as the film coefficient. This is a laminar film of resistance to heat flow that exists on both sides of the heat transfer surface. Lower coolant temperature has a negligible effect on the film and increased fluid flow rates would incur serious operating cost penalties.
U.S. Pat. No. 3,127,753 discloses a system in which a suitable refrigerant, such as Freon 12 (dichlorodifluoromethane) (Freon is a trademark of E. I. DuPont de Nemours & Co.), is delivered in liquid state to a single expansion valve. When passing through the expansion valve, some of the liquid refrigerant changes to vapor. A mixture of liquid and vapor enters each mold die element through single feed lines. In the mold die elements, the refrigerant is changed to superheated vapor which then exits to a conventional compressor, condenser, and receiver arrangement for recirculation. A superheat control is provided to modulate the single expansion valve and ensure that all refrigerant exiting the mold dies is superheated. The patent also discloses a means for controlling the degree of mold cooling comprising thermocouples in the mold die elements associated with modulating valves in the single mold die feed lines.
Thus, U.S. Pat. No. 3,127,753 addresses itself to increasing system efficiency by eliminating a separate heat exchanger and by lowering the coolant temperature. The apparatus disclosed therein is intended to make the mold as cold as possible. This means that the room must be dehumidified and capital equipment must be added to obtain the lower temperatures. Coolant compression costs are much greater for colder vapor since larger compression equipment is needed to remove a given amount of heat at a lower refrigerant temperature.